Delamination Damage to Composite Materials Caused by Radiant Heating
The objective of this investigation was to quantify the damage to a series of aerospace composite materials exposed to controlled radiant fluxes over periods ranging from seconds to minutes. Physical damage to the composite plates during heat exposure, as well as, the plate response itself, was documented with regular video, infrared video, and post-test imaging, sectioning and microscopy. Temperature in the plates was monitored with embedded thermocouples (TC). Both four-point bend and tensile testing was conducted to quantify the mechanical degradation due to heat exposure.
For all materials and heat flux intensities tested, it was found that the composite materials experienced sudden and catastrophic damage in the form of delaminations, sometimes throughout the entire thickness, prior to any significant charring or mass loss. Delaminated composite material samples displayed little residual mechanical strength in flexure (up to 85-percent strength loss) and greatly reduced tensile strength (up to 40-percent strength loss). In contrast, samples exposed to similar heat flux and durations, but removed from heat prior to delamination, showed little reduction in mechanical strength. The time at which delamination occurred was indicated by thermocouple data and global buckling of the plate. Time to delamination was affected by the moisture content of the plate, with delamination occurring at shorter exposure times for plates with higher moisture content.
Further heat exposure tests should be conducted at varied heat flux and water moisture levels. Heat exposure tests should also be expanded to include the following: (1) testing at additional water moisture levels, laminate thickness and layups; (2) testing in a mechanically-constrained condition to inhibit global buckling in order to examine the effect on damage in the plates; (3) testing under load. Mechanical stresses due to loading may cause the heat-induced delaminations seen during testing to occur at even lower temperatures. Finally, a model needs to be developed to predict the thermally-induced delaminations seen in the experiments.
The increasing use of composite materials and other specialized U.S. Air Force and DoD materials in aircraft and weapons systems intensifies the challenges to firefighters. Relatively small fires on composite aircraft, from for example a nacelle fire or wheel brake fire, can still lead to millions of dollars of heat-induced damage to surrounding structures. In order to minimize the cost of these fires, firefighting response requirements need to be estimated to establish concepts of operations which will permit these types of fires to be routinely extinguished while the level of damage is still small. The data from this investigation is necessary for determining response requirements.